The use of backscattered light in fiber optic cables has found increasing acceptance in a variety of applications. Because light can be backscattered from any location along the length of a fiber, information can be obtained over significant distances and such systems are often referred to as “distributed” sensors. Because distortion or deformation of the fiber can be sensed, distributed sensors comprised of fiber optic cable can be used to sense temperature, pressure, strain, acoustic events, and the like. Distributed systems have been used advantageously in oilfield applications, in traffic monitoring, and in military/security applications, among others.
In a typical fiber optic-based distributed sensing system, one or more fiber optic cables designed to collect distributed strain measurements are deployed in a desired location and coupled to the sensing subject by suitable means. One or more light boxes containing laser light sources and signal-receiving means are optically coupled to the fiber. In some embodiments, the light source may be a long coherence length phase-stable laser and is used to transmit direct sequence spread spectrum encoded light down the fiber. The cable may be double-ended, i.e. may be bent in the middle so that both ends of the cable are at the source, or it may be single-ended, with one end at the source and the other end at a point that is remote from the source. The length of the cable can range from a few meters to several kilometers, or even hundreds of kilometers. In any case, measurements can be based solely on backscattered light, if there is a light-receiving means only at the source end of the cable, or a light receiving means can be provided at the second end of the cable, so that the intensity of light at the second end of the fiber optic cable can also be measured.
When it is desired to make measurements, the light source transmits at least one light pulse into the end of the fiber optic cable and a backscattered signal is received at the signal-receiving means. Localized strain or other disruptions cause small changes to the fiber, which in turn produce changes in the backscattered light signal. The returning light signal thus contains both information about the deformation of the fiber and location information indicating where along the fiber it occurred. Known optical time-domain reflectometry (OTDR) methods can be used to infer information about the sensing subject based on the backscattered signal from one or more segments of the fiber adjacent to the subject. Typically, the location of the backscattering reflection at a point along the fiber can be determined using spread spectrum encoding, which uniquely encodes the time of flight along the length of the fiber, dividing the fiber into discrete channels along its length.
In some applications, including downhole applications, the physical channel depths cannot practically be measured directly, but they can be roughly inferred on the basis of timing and fiber refraction index, i.e. the “optical depth.” These rough calculations are not sufficiently precise for some purposes, however, because they incorporate uncertainties that, while small on a percent scale, build to a significant magnitude over the length of the fiber. For downhole seismic applications, repeatable physical depth positioning of the channels within an accuracy of 1 meter or better is desired.
Currently, however, there is no practical way to determine the actual physical location of a given backscattered signal. Hence, there remains a need for a system that would allow the physical location of a given backscattered signal to be determined with the desired accuracy and, if possible without requiring re-entry of the well at a later date to measure channel drift.